The aim of the proposed research is to provide a quantitative description of how reflex movements are organized, temporally and with regard to the relative use of synergistic muscles. The smallest element of this organization is the motor unit. The relative excitability of motor units of different types will be studied as a function of time, as these units are activated in a variety of reflexes. Organization of movements will be described in terms of recruitment order of motor units among the several muscles activated. Muscle response to reflex activation will be studied at four levels: 1. Order of activation of motor units of one muscle as it first responds to a reflex input: reflexes of varying strength will be used. 2. Time course of motor unit response: the functional significance of EPSP-IPSP complexes in motoneuron response will be studied. 3. Time course of response of synergists and antagonists: the effect of reflex inputs on the time sequence of activation of motoneurons of several muscles will be studied. The result will be a description of how motor units of different size and type are utilized in each of several reflex movements. 4. Firing patterns of motor units of different type: Statistical analysis of firing patterns of motor units of different types will be studied, to determine how these different units are controlled by complex reflex action, and by willed movements.